Speeds of central processing units (CPUs), dynamic random access memories (DRAMs), static random access memories (SRAMs), etc. have increased with the development of silicon semiconductor technology. However, while semiconductor chip operating speeds have increased, communication speeds between semiconductor chips have remained stagnant. A method of performing communication between chips using light has been suggested to solve this problem.
Methods of connecting chips using photoelectric devices using compound semiconductors have been studied in all the world. However, compound semiconductor photoelectric devices are expensive, and use advanced package technology. There are methods of integrating silicon photoelectric devices into silicon semiconductor chips to ease fabrication and reduce cost. An optical modulator will be described as an example of such a silicon photoelectric device.
There are several types of silicon optical modulator. The first uses resonance and/or anti-resonance of a Fabry-Perot Cavity. This is described in detail in Journal of Lightwave Technology, Vol. 21, No. 4, pp. 1089-1098, 2003, ‘Low-Power Consumption Short-Length and High-Modulation-Depth Silicon Electrooptic Modulator.’ In this type, the amount of light passing the Fabry-Perot Cavity is adjusted according to a current applied to a material between two mirrors of the Fabry-Perot Cavity, e.g. silicon. However, the characteristics of the Fabry-Perot Cavity change with temperature.
A silicon semiconductor chip must normally operate over a range of about 100° C., and thus any integrated silicon photoelectric device must match this. However, since the refractive index of silicon changes with temperature, the amount of light passing the Fabry-Perot Cavity varies. In other words, in the Fabry-Perot Cavity, the currents corresponding to resonance and anti-resonance are valid only at a specific temperature. Thus, the chip temperature must be sensed to vary the currents with the temperature so as to apply accurate resonance and anti-resonance currents. A method of using a Mach-Zehnder interferometer is employed to solve this temperature problem.
FIG. 1 is a schematic view of a Mach-Zehnder optical modulator. Referring to FIG. 1, the Mach-Zehnder optical modulator includes an input optical waveguide 1, an output waveguide 2, and two branches 3 and 4 connected between the input and output optical waveguides 1 and 2. The operation of the Mach-Zehnder optical modulator will now be briefly described. Light is input to the input optical waveguide 1 and split into two light portions at a first branch point 5. The two light portions pass through the two branches 3 and 4 and then are combined at a second branch point 6. The combined light is output to the output waveguide 2. If any variation is not applied to the two branches 3 and 4, constructive interference occurs due to the combination of the two light portions at the second branch point 6. Thus, the intensity of the original light is maintained. However, if a current or a voltage is applied to the branch 3 to change the refractive indexes of an optical waveguide, a phase difference occurs between the two light portions. Thus, when the two light portions are combined at the second branch point 6, destructive interference may occur. As a result, constructive or destructive interference may occur depending on whether an electrical signal is applied to a branch. This allows light to be modulated.
Since the refractive indexes of the two branches vary with temperature in this method, light portions passing the two branches undergo the same phase shift with the change of the temperature. Thus, a phase difference can occur between the two light portions only if an electric signal is applied to one of the two branches. As a result, the state of constructive or deconstructive interference is not dependant on the temperature.
The most advanced characteristic of such a Mach-Zehnder silicon optical modulator is described in Optics Express, vol. 13, No. 8, pp. 3129-3135 , 2005, ‘High Speed Silicon Mach-Zehnder Modulator’ and Nature, Vol. 427, No. 12, pp. 615-618, 2004, ‘A High Speed Silicon Modulator based on Metal-Oxide-Semiconductor Capacitor.’ Two arms of a Mach-Zehnder interferometer constitute a metal-oxide-semiconductor (MOS) structure in this method. The MOS structure uses a method of applying a voltage to accumulate charges on both sides of an oxide layer and vary the refractive index of silicon using the accumulated charges. Here, the refractive index Δn is varied as in Equation 1 when an input wavelength is 1.55 μm.Δn=−[8.8×10−22×ΔN+8.5×10−18×(ΔP)0.8]  (1)
Wherein ΔN and ΔP respectively denote variations of concentrations of electrons and holes. As shown in Equation 1, the variation of the refractive index is greater when the variation of the concentrations of electrons and holes are great. However, since the variation of refractive index caused by an applied current in silicon is smaller than that of a compound semiconductor, the variation of the applied current must be great. The area in which the concentrations of electrons and holes vary should be wide. A confinement factor Γ should be great to increase the variation of refractive index. The confinement factor Γ is defined as the intensity of light passing a portion in which a refractive index varies with respect to an entire intensity of light.
A variation of a substantial effective refractive index is ‘Δneff=Γ×Δn.’ However, the confinement factor Γ is small in the MOS structure. As a result, the variation of the effective refractive index is small. This makes it difficult to constitute an effective Mach-Zehnder optical modulator.
This will be described in more detail. If a voltage is applied to the MOS structure, charges are generated as in Equation 2:ΔN=ΔP=∈0×∈r×(V−VFB)/(tox×t)  (2)
wherein ∈0 denotes the permittivity of a vacuum, ∈r denotes the dielectric constant of a gate oxide, VFB denotes a flat band voltage of the MOS structure, tox denotes the thickness of the gate oxide, and t denotes the thickness of a portion in which charges are gathered. Typically, tox is about 5 nm, and t is 10 nm. If constants of materials and these figures are substituted for Equation 2, the amount of charges is ‘ΔN=ΔP=˜4×1018×(V−VFB)cm−3.’ Here, since ‘(V−VFB)’ is about 2 V, ‘ΔN=ΔP’ is about 1019 cm−3. However, since the confinement factor r is only several percent, the effective refractive index variation Δneff is not great. Thus, it is difficult to effectively vary the refractive index due to a small confinement factor in a MOS structure. As a result, it is difficult to greatly vary the phase of light. Therefore, the arms of a Mach-Zehnder modulator become long. The length of each arm of a Mach-Zehnder modulator is several millimeters in the above-mentioned paper. The long length of the arm additionally increases capacitance and thus makes high-speed modulation difficult. Also, a gate part is generally formed of polysilicon in the MOS structure. The polysilicon well absorbs light and thus reduces efficiency.
There is a method of injecting a current using a p-i-n structure, besides a MOS structure. This method is disclosed in detail in U.S. Pat. No. 5,908,305. When this method is used for silicon, modulation speed is limited. After a current is injected into the p-i-n structure, injected charges recombine and thus disappear. Here, the average time required for recombining the injected charges is very long in the case of silicon. Thus, a long time is required to return to their original states. This limits the modulation speed.
There has been suggested a method of operating a part of which phase shifts in a PN reverse bias mode to solve the problems of the MOS structure or the p-i-n structure. An example of this method is disclosed in U.S. Pat. No. 2006/0008223 A1. In this patent, a silicon lateral PN diode is formed in a rib optical waveguide to operate in a PN reverse bias mode so as to constitute a Mach-Zehnder optical modulator. However, Improvement of confinement factor and optimization of doping concentration need.